1. Field of the Invention
The invention relates to thermoplastic resin compositions and more particularly relates to improved, blow-moldable copolyester-carbonate resins and articles molded therefrom.
2. Brief Description of the Prior Art
Aromatic copolyester-carbonate resins are a well known class of synthetic polymeric resins, generally prepared by the reaction of a polyhydric phenol with a carbonate precursor in the presence of an ester precursor; see for example U.S. Pat. No. 3,169,121. Although such resins have been found to be thermoplastically moldable under a broad range of molding conditions, only select copolyester-carbonate resin compositions are useful for blow-molding. This is due to the unique requirements of a thermoplastic resin for blow-molding operations; see for example the requirements for the branched copolyester-carbonate resins described in U.S. Pat. Nos. 4,286,083 and 4,621,132. The branched copolyester-carbonate resins differ from most thermoplastic polymers used for molding in their melt rheology behavior. Most thermoplastic polymers exhibit non-Newtonian flow characteristics over essentially all melt processing conditions. However, in contrast to most thermoplastic polymers, certain branched copolyester-carbonates prepared from dihydric phenols exhibit Newtonian flow at normal processing temperatures and shear rates below 300 reciprocal seconds.
Newtonian flow is defined as the type of flow occurring in a liquid system where the rate of shear is directly proportional to the shearing force.
Two other characteristics of molten thermoplastic polymers are considered to be significant for molding operations: melt elasticity and melt strength. Melt elasticity is the recovery of the elastic energy stored within the melt from distortion or orientation of the molecules by shearing stresses. Melt strength may be simply described as the tenacity of a molten strand and indicates the ability of the melt to support a stress. Both of these characteristics are important in extrusion blow molding, particularly in fabrication by extrusion blow molding of relatively large articles. Non-Newtonian flow characteristics tend to impart melt elasticity and melt strength to polymers thus allowing their use in blow molding fabrication.
In the conventional blow-molding operation, a tube of the heat-softened copolyester-carbonate resin may be extruded vertically into a mold. The extrudate is then pressed unto the mold surfaces with a pressurized gas flow (usually air or inert gas), shaping the heat-softened resin. As mentioned above, the successful molding of a given thermoplastic resin is dependent upon a number of factors, including the characteristics and physical properties of the heat-softened resin.
However, even though a given branched copolyester-carbonate resin may have the physical properties required for successful blow-molding, the product articles may be deficient in certain other physical properties otherwise desired. For example, the molded articles may lack a desired degree of impact strength, particularly at low temperatures. We have found that polyester-carbonate resins of a particular class are represented by a branched polyester-carbonate composition that exhibits a lowered glass transition (Tg) temperature and the shear-thinning behavior of a blow-molding grade resin. The Tg of the resin is reduced by the presence of aliphatic diester blocks; while the branching is obtained by using a particular class of trisphenol.
The non-Newtonian rheological behavior and reduced glass transition temperature of this resin has use in injection molding. An example of such an application is in molded computer and business equipment housings. These large and complex parts require materials with reduced viscosities at shear rates experienced during mold filling in the injection molding process. The compositions of the invention provide the type of rheological behavior sought in this use. The copolymers, based on bisphenol-A, an alkanedioic acid, and 1,1,1-tris-(4-hydroxyphenyl)ethane, display improved processability at lower processing temperatures as well as the typical non-Newtonian behavior of a branched polycarbonate homopolymer. Advantages include extrusion at lower temperature, reduced torque, smoother surface appearance of the extruded parts, and ability to coextrude with heat sensitive material that generally cannot be processed at conventional polycarbonate molding temperatures.
Melt processable copolyester-carbonates having relatively high glass transition temperatures (on the order of 180.degree. C. or more) are described in the U.S. Pat. No. 4,310,652 (DeBons et al., Jan. 12, 1982). The term "low glass transition temperature" 1982). The term "low glass transition temperature" or "reduced Tg" as used in the present invention means a Tg of less than 145.degree. C.
The U.S. Pat. No. 4,621,132 (Quinn et al., Nov. 4, 1986) describes branched copolyester-carbonate resins, including resins randomly branched with trisphenol branching agents. The resins are prepared from an aromatic ester precursor (isophthaloyl dichloride).